Williams.] PRINCIPLES OF CORRELATION. 13 
the Hamilton formation have made it possible to treat the facts con- 
cerning it with a degree of precision that would not be possible in con- 
sidering a formation which is less perfectly known or one the facts 
concerning which are scattered and but imperfectly classified. 
For the discussion of a geological fauna it is also important to have 
some conception of the environmental conditions under which it lived 
and the succession of conditions which have preceded and led up to 
them. Thus, to understand the fossil fauna preserved in the Hamil- 
ton formation, it is needful to reconstruct the physical conditions of 
the Devonian sea in which the fauna lived, and to look backward 
over the history of that sea for some considerable period of geological 
time. In order to describe a fossil fauna it must be traced back to a 
time when it was not, and onward till it has ceased, and thus the his- 
tory of the basin in which the evolution has taken place is incidental 
to the description of the fauna itself. 
DEFINITIONS AND NOMENCLATURE OF FAUNAL PALEONTOLOGY. 
The primary fact that fossils may be used in identifying formations 
and tracing them from place to place was announced and demonstrated 
by William Smith. Many other laws regarding the order and suc- 
cession of fossils have been formulated by d'Archiac, Bronn, Pictet, 
Lyell, Brongniart, Zittel, and other writers on paleontology. But in 
addition to these fundamental and established laws of the relations of 
fossils to formations, there are some special facts or principles per- 
taining to the relations which living organisms bear to their environ- 
ment and to each other, brought out by the study of organic evolution, 
which require definitions and lead to the adoption of terms differing 
somewhat from those in common use, at least with special application 
to correlation and the expression of time relations in geology. 
The question here raised is not, Can geologic formations be corre- 
lated by their contained fossils? The fact of correlation is taken for 
granted; but the questions are, Wherein does correlation consist? 
W r hat is done in correlation? Upon what principle are correlations 
made? 
Thus the discriminations to be made pertain to the relations which 
fossils bear to one another, to the geological conditions of preserva- 
tion, to the conditions of their living and continuing to live in the 
past, and, finally, to the value of fossils as means of distinguishing 
different periods of geological time as well as of identifying like periods 
of time represented by them. 
ANIMAL AND PLANT AGGREGATES. 
To discuss organisms in their relations to time, it becomes necessary 
to treat of them in aggregates and to discriminate the reasons for 
which the particular aggregations are made. 
The zoologist associates organisms on the basis of their morpholog- 
ical affinities, and calls the aggregates species, genera, orders, etc. Two 
